I'm new to feeding horses... Got a 20 year old Morgan in February and went through a couple feed combinations before I settled on orchard grass and Purina Equine Senior. He gets about 3.5 pounds of the Senior twice a day (7-8 pounds a day total) and the orchard grass is pretty much free choice.... It seemed he never ran out.

Then I switched suppliers. Now he hits that hay a lot harder. The stuff he never ran out of looked a little drier, didn't have visible seed (or flower?) heads in the bale, and the bales felt lighter too me. The new stuff is greener, has apparent seed or flower heads in it, weighs more, seems to generate more energy in him, and he cleans his feed bin. I'm paying more for it.

Am I likely to be correct in thinking the orchard grass he likes better is actually better hay? He eats it well, so that right there scores points for an older horse.... but only if he doesn't get chubby I guess. He isn't.... yet.

I haven't switched anything about the senior.

He goes about 4-5 miles every other day in hilly trail terrain carrying one of my 70 pound kids. About every other week we go for a 10 mile day in hilly terrain.

I appreciate thoughts....

Your right, finding a diet that works for one particular horse is trial an error.
Yes, from what you describe you are feeding a better quality hay.
Feeding can be as simple as hay/grain/water or a serious scientific study.
Some points to consider;
Hay should be your primary source of fiber & carbohydrates
Good quality provides protiens,vitamins,minerals and especially calcium
Type-Legumes are higher in protien & more palatable
Soil conditions-Drought can affects quality
Growth-Hay should be cut before mature,they are less stemmy and more nutritious
Non Nutritious matter-Hay should be clean and free of weeds, toxic plants or foreign objects
Quality-Can change from each cutting or field produced in. Should look and smell fresh and have a close to green color.
A legume is round leaved, grasses are straight leaved
Calories in hay can range from Rye grass @ .23 Mcal to Birdsfoot trefoil @ 1.20Mcal
Protein Timothy @ 5% to Alfalfa @ 22%
Fiber runs fairly close to 5-7% except Brome @ 19 %, I've found horses don't seem to like it.
A horse @ work needs enough nutrients to meet his maintenance requirements, plus enough energy for the work he does and to provide formaintenance and repair of tissues under increased stress. Protien and calcium requirements increase slightly with intense work.
Your horse would be considered in light work;
Light work-nonstressful, fairly slow, about 30 minutes to 1 hour per day. Light work is performed mostly @ walk with 25% @ trot and 15% @ canter.
Moderate work-A medium level of exercise, about 1-2 1/2 hours per day. Moderate work is performed mostly @ trot and canter, with some walking and galloping.
If you want to go farther;
Walking- .5 kcal/hour
Slow trotting, some canter- 5.0 kcal/hour
Trotting/cantering /jumping- 12.5 kcal/hour
Cantering/jumping/galloping- 23.0 kcal/hour
Racing/polo/eventing- 39.0 kcal/hour

But to answer your original question, Yes this hay is probably better.
If he readily finishes it that is a good thing.
Watch his weight, both eyeball him and a weight tape is helpful.
If you find he is gaining to much weight reduce your commercial grain, as he can get all his nutrients from hay.
If your in a climate that gets cold during the winter, extra hay, the fermentation breakdown is what creates body heat.

Great question.

I am going to need to spend some time with all the information in that post! Thank you very much.

OK. I've been thinking about this information.

First thought: I can't afford to play polo.

I found a webiste that says orchard grass can be expected to deliver 870 calories per pound....

I found nothing about the caloric value of Purina Eqiune Senior, neither on the web nor the bag tag. However the bag says a 1000 pound horse needs 14.75 pounds per day for maintenance, so trying to work backward from that number.... if a 1000 pound horse needs 15K cals per day for maintenance, then 15000/14.75 = @1000 cals per pound.

So, I have been giving him 7-8 lbs per day of the senior, for about 7-8000 calories, plus 10-12 pounds of Orchard grass per day, for 8700-10,440.... for a total of 15,700 - 18,440 cals per day.... and he has stayed roughly the same weight with the workout schedule I described.

It seems like there is a pretty huge difference between .5 Kcal per hour at a walk and 5Kcal per hour at a slow trot or canter. That is something to keep in mind. Walking versus trotting could be equated to a factor of ten times more calorically demanding. Which causes me to ask, what about the effect of hills, while walking. We have virtually no flat ground where I live. We are always either going up or down. I live in the Sierra Nevada foothills. Our 4 miles rides probably see about 1000' of elevation gain but our 10 mile rides may be 3000' elevation gain and loss.

After some of our ten mile days in hills I have presumed he was lethargic for a few days because he needed a few days muscle recovery, which is probably not completely incorrect, but I bet now that his recovery time may have been reduced by increasing his calorie intake. Any thoughts on this and how to do it if you think I may be right?

Maybe it should be a different thread, or maybe I need to search and read a little before I ask a stupid question that has been asked and answered a million times, but I am also sort of wrapping my mind around an examination of the value of 1)electrolyte supplementation (we sometimes go out for a 4 miler at 90 degrees because it is cooler than the 105 degree high for the day), and 2)glucosamine....

Don't worry, I don't do Polo either. Way to strenuous.

I calculate my feeding charts differently, and don't start w/ calories but TDE [total digestable energy] But your #'s come out close, so it looks like two different way to the same answer.

No, When reading a commercial feed label, if you really read it, gives little to no useful info.

I use;
horse x recommended hay=hay ration
horse x recommended grain= grain ration
Add hay + grain = TDR
lbs TDR x Mcal = Mcal/day-Energy
There is a lengthy chart for this, called the Morris Feeding Standards

Absolutely correct. All of the data that is out there has been collected from controlled studies on flat ground on manicured footing. So anything other than that plays into calories expended.

There is some legitimacy to caloric intake and recovery rate. I feed all of my horses even the aged ones 8-12% protien, over-use of protien is known to increase body fat, to reduce speed , recovery and exercise tolerance, as well as causing a horse to Tie up[Azoturia] and may contribute to more common muscle tearing
But in your case it sounds as if you are dealing with;
A. An older horse
B. Constant hilly terrain
C. Light workload
With those in combination, I would not be overly concerned, and be looking for a diet change straight away.

Supplements are a multi billion dollor industry, we feed them because it makes Us feel better. If you decide to add a supplement you will need to factor in the vitamin/mineral content, most fed as per label are fine. Using too many minerals and supplements risk absorption problems and also deficiences.
Go with the old saying-If it ain't broke, don't fix it.

Sounds as if your on the right track. The exact amounts depends on each individual horse, his metabolism and workload.
Simply, the diet must provide;
enough energy for level of exercise
protien to maintain body structures and repair damaged tissues